Semiconductor device

ABSTRACT

The NMOS transistor circuit has the surge protection circuit connected in parallel with the NMOS transistor. Resistor is provided between back gate of the NMOS transistor and the ground. As a result, input impedance higher than input impedance of the surge protection circuit is applied to a semiconductor terminal at the electrode pad side of the NMOS transistor.

FIELD OF THE INVENTION

[0001] The present invention relates to a semiconductor device that prevents electrostatic breakdown of a metal oxide semiconductor (MOS) transistor within a semiconductor integrated circuit.

BACKGROUND OF THE INVENTION

[0002] Countermeasure against breakdown due to static electricity is important for protecting the high reliability of the semiconductor integrated circuits. On the other hand, it has become practically difficult to achieve this countermeasure against breakdown due to static electricity because of miniaturization and complicatedness of the semiconductor integrated circuit in recent years.

[0003] Particularly, generally, the semiconductor integrated circuits include MOS transistors. Among others, the input of a CMOS (complementary metal oxide semiconductor) transistor consists of high-insulation electrode and another electrode via a thin high-insulation film covered on this insulated electrode. A capacitor is disadvantageously formed, between these electrodes of the transistor, which may be sometimes electrically charged. If this capacitor is electrically charged, it momentarily outputs a high voltage to the outside via an electrode pad connected to the transistor.

[0004] Similarly, switching operation of mechanical switches or semiconductor switches provided outside of the semiconductor integrated circuit may cause a high voltage to be momentarily applied to the electrode pad connected to the transistor.

[0005] When such momentarily generated high voltage is applied to transistors or other external elements as described above, what is called a surge breakdown is generated like the breakdown of these transistors and the reduction of their life. To avoid this problem, usually a surge protection circuit is disposed between the transistor and the electrode pad of this transistor.

[0006]FIG. 5 is a circuit diagram of NMOS (N-channel metal oxide semiconductor) transistor circuit as an example of semiconductor devices. This transistor circuit 100 includes NMOS transistor MN10 and surge protection circuit 101. The NMOS transistor MN10 has its drain (source) connected to the electrode pad 102, and has its source (drain) and back gate connected to the ground. The surge protection circuit 101 is connected in parallel with the NMOS transistor MN10 as concerned to the electrode pad 102. Further, the surge protection circuit 101 includes two diodes connected in series. Cathode of one of these diodes is connected to a power source terminal, and anode of the other diode is connected to the ground. Node of these diodes is connected to the electrode pad 102.

[0007]FIG. 6 is a cross-sectional construction diagram for explaining the operation of the NMOS transistor shown in FIG. 5. Consider that surge voltage is applied to the electrode pad 102. As a result, surge current flows into the surge protection circuit 101, and therefore the NMOS transistor MN10 is not destroyed. Thus, surge breakdown of this transistor is prevented.

[0008] However, as shown in FIG. 6, when the input impedance of the NMOS transistor MN10 at the electrode pad 102 becomes lower than the input impedance of the surge protection circuit 101 due to the connection of a back gate P⁺ diffusion layer of the NMOS transistor MN10 to the ground, the surge current pierces through a surface of a junction between a drain (source) N diffusion layer 112 and a P⁻ well 113. The surge current then passes through a back gate P⁺ diffusion layer 111 and a P⁻ substrate 114 to reach the ground. This brings about electrostatic breakdown of the junction surface.

[0009] In order to prevent the electrostatic breakdown, conventionally, a transistor of bigger size is used thereby increasing the backward withstand voltage between the drain (source) N diffusion layer 112 and the P⁻ well 113.

[0010] However, there is a problem that, along with the reduction in the sizes of transistors based on the reduction in the chip sizes and the fining of the processes following this trend, the drain (source) diffusion layer has become thinner. As a result, it has become difficult to sufficiently obtain the surge withstand voltage.

SUMMARY OF THE INVENTION

[0011] It is an object of this invention to provide a semiconductor device capable of preventing the electrostatic breakdown even when the semiconductor device is composed of sufficiently fine MOS transistors.

[0012] The semiconductor device according to one aspect of this invention includes a MOS transistor having a source or a drain connected with a wire to an electrode pad; a surge protection circuit connected in parallel with the wire that connects the MOS transistor and the electrode pad; and an impedance adding unit that has input impedance higher than input impedance of the surge protection circuit and that is connected to a back gate of the MOS transistor.

[0013] According to the above-mentioned aspect, in a MOS transistor circuit having a surge protection circuit connected in parallel with a MOS transistor, the impedance adding unit is provided at the back gate of the MOS transistor. Therefore, it is possible to apply an input impedance higher than an input impedance of the surge protection circuit to between the back gate of this MOS transistor and the ground (or the power source terminal).

[0014] The semiconductor device according to another aspect of this invention includes a plurality of MOS transistors and same number of electrode pads as the MOS transistors, wherein each of the MOS transistor having a source or a drain connected with a wire to corresponding the electrode pad; same number of surge protection circuits as the MOS transistors, wherein each of the surge protection circuit being connected in parallel with the wire that connects corresponding the MOS transistor and the electrode pad; and an impedance adding unit that has input impedance higher than input impedance of each of the surge protection circuit and that is connected in common to back gates of the MOS transistors.

[0015] According to the above-mentioned aspect, in a plurality of MOS transistor circuits having a surge protection circuit connected in parallel with each MOS transistor, the impedance adding unit connected in common to the back gate of each MOS transistor is provided. Therefore, one impedance adding unit can apply an input impedance higher than an input impedance of the surge protection circuit to between the back gate of each MOS transistor and the ground (or the power source terminal).

[0016] The semiconductor device according to still another aspect of this invention includes a MOS transistor having a source or a drain connected with a wire to an electrode pad; and a surge protection circuit connected in parallel with the wire that connects the MOS transistor and the electrode pad, wherein back gate of the MOS transistor is connected to a point of a semiconductor circuit at which point the impedance is higher than input impedance of the surge protection circuit.

[0017] According to the above-mentioned aspect, in a MOS transistor circuit having a surge protection circuit connected in parallel with a MOS transistor, a high impedance point of other semiconductor circuit is connected to a back gate of the MOS transistor. Therefore, it is possible to apply an input impedance higher than an input impedance of the surge protection circuit to between the back gate of this MOS transistor and the ground (or the power source terminal).

[0018] Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1A and FIG. 1B are diagrams showing a CMOS transistor circuit as a semiconductor device relating to a first embodiment of the present invention;

[0020]FIG. 2 is a cross-sectional construction diagram of a CMOS transistor circuit for explaining the operation of the semiconductor device relating to the first embodiment;

[0021]FIG. 3 is a diagram showing an NMOS transistor circuit as a semiconductor device relating to a second embodiment of the present invention;

[0022]FIG. 4 is a diagram showing an NMOS transistor circuit as a semiconductor device relating to a third embodiment of the present invention;

[0023]FIG. 5 is a diagram showing an NMOS transistor circuit as one of conventional semiconductor devices; and

[0024]FIG. 6 is a cross-sectional construction diagram for explaining the operation of the NMOS transistor circuit as one of conventional semiconductor devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Preferred embodiments of the present invention will be explained in detail below with reference to the accompanying drawings. The present invention is not limited to these embodiments.

[0026] The semiconductor device relating to a first embodiment of the invention will be explained now. FIG. 1A and FIG. 1B are circuit diagrams of CMOS transistor circuit, as an example of semiconductor devices, relating to the first embodiment. Particularly, FIG. 1A shows NMOS transistor circuit and FIG. 1B shows PMOS transistor circuit that make the CMOS transistor circuit.

[0027] The NMOS transistor circuit 10 shown in FIG. 1A includes NMOS transistor MN, surge protection circuit 1, and resistor R1. The NMOS transistor MN has its drain (source) connected to the electrode pad 2, has its source (drain) connected to the ground, and has its back gate connected to one terminal of the resistor R1. The surge protection circuit 1 is connected in parallel with the NMOS transistor MN as concerned to the electrode pad 2. As shown in FIG. 1A, the surge protection circuit 1 consists of two diodes D1 and D2 connected in series. Cathode of one diode D1 is connected to a power source terminal, and anode of the other diode D2 is connected to the ground. Node of the diodes D1 and D2 is connected to the electrode pad 2. The other terminal of the resistor R1 is connected to the ground.

[0028] The PMOS transistor circuit 20 shown in FIG. 1B includes PMOS transistor MP, surge protection circuit 3, and resistor R2. The PMOS transistor MP has its drain (source) connected to an electrode pad 4, has its source (drain) connected to a power source terminal, and has its back gate connected to one terminal of the resistor R2. The surge protection circuit 3 has the same structure as that of the surge protection circuit 1, and the node of the two diodes is connected to the electrode pad 4. The other terminal of the resistor R2 is connected to the power source terminal. The resistors R1 and R2 are polysilicon resistors or diffusion resistors.

[0029]FIG. 2 is a cross-sectional construction diagram of the CMOS transistor circuit for explaining the operation of the semiconductor device relating to the first embodiment. In FIG. 2, the complementary connected structure of an NMOS transistor MN and a PMOS transistor MP is the same as the conventional structure, and therefore, the explanation of this construction will be omitted here. P⁻ well 13, N⁻ well 23, and P⁻ substrate 7 are separated by N⁺ floating layer 6, and potential of back gate can be set optionally.

[0030] When a surge voltage is applied to the electrode pad 2, surge current tries to flow to the surface of a junction between a drain (source) N diffusion layer 12 of the NMOS transistor MN and P⁻ well 13. However, because of the existence of the resistor R1, the impedance of the back gate of the NMOS transistor MN becomes higher than that of the surge protection circuit 1. As a result, the surge current flows to the surge protection circuit 1. Therefore, based on this arrangement, it is possible to prevent the occurrence of the electrostatic breakdown on the surface of the junction.

[0031] Similarly, when a surge voltage has been applied to the electrode pad 4, a surge current tries to flow to the surface of a junction between a drain (source) P diffusion layer 22 of the PMOS transistor MP and an N⁻ well 23. However, because of the existence of the resistor R2, the impedance of the back gate of the PMOS transistor MP becomes higher than that of the surge protection circuit 3. As a result, the surge current flows to the surge protection circuit 3. Therefore, based on this arrangement, it is possible to prevent the occurrence of the electrostatic breakdown on the surface of the junction.

[0032] As explained above, according to the semiconductor device relating to the first embodiment, in a MOS transistor circuit having a surge protection circuit connected in parallel with a MOS transistor, a resistor is disposed between the back gate of this MOS transistor and the ground (or the power source terminal). Based on this arrangement, an input impedance higher than that of the surge protection circuit is applied to the semiconductor terminal at the electrode pad side of the MOS transistor. Therefore, it is possible to prevent a surge current from flowing via the back gate of the MOS transistor. As a result, it becomes possible to prevent the MOS transistor from being destroyed and short-lived.

[0033] The semiconductor device relating to a second embodiment of the invention will be explained now. The semiconductor device relating to the second embodiment is characterized in that a high-impedance circuit is provided in place of the resistor R1 shown in FIG. 1A.

[0034]FIG. 3 is a circuit diagram of NMOS transistor circuit, as an example of semiconductor devices, relating to the second embodiment. In FIG. 3, portions common to those in FIG. 1A are provided with like reference numbers, and their explanation will be omitted. In the NMOS transistor circuit 40 shown in FIG. 3, high-impedance circuit 41 is connected to back gate of the NMOS transistor MN.

[0035] The high-impedance circuit 41 includes the pnp transistor 43 having its emitter connected to the constant-current source 42, having its base connected to the constant-voltage source 44, and having its collector connected to the ground. Particularly, in this high-impedance circuit 41, the emitter of the pnp transistor 43 is connected to a back gate of the NMOS transistor MN. In other words, the impedance of the emitter resistor of the pnp transistor determined by a constant current supplied from the constant-current source 42 and a constant voltage applied by the constant-voltage source 44 exists between the back gate of the NMOS transistor MN and the ground.

[0036] As explained above, according to the semiconductor device relating to the second embodiment, in a MOS transistor circuit having a surge protection circuit connected in parallel with a MOS transistor, a high-impedance circuit is disposed between the back gate of this MOS transistor and the ground (or the power source terminal). Based on this arrangement, an input impedance higher than that of the surge protection circuit is applied to the semiconductor terminal at the electrode pad side of the MOS transistor. Therefore, it is possible to obtain the effects similar to those of the first embodiment. Particularly, the employment of this high-impedance circuit is effective when there is no space for building a polysilicon resistor or a diffusion resistor on the chip.

[0037] The above high-impedance circuit may be any circuit that has a portion of impedance higher than that of the surge protection circuit. Particularly, when there exists a high-impedance portion in other circuit on the same chip on which the MOS transistor is formed, it is possible to draw this high-impedance portion to the back gate of the MOS transistor as a high-impedance line.

[0038] The semiconductor device relating to a third embodiment of the invention will be explained now. The semiconductor device relating to the third embodiment is characterized in that the semiconductor device has a resistor connected in common to back gates of respective MOS transistors of a plurality of MOS transistor circuits.

[0039]FIG. 4 is a circuit diagram of NMOS transistor circuit, as an example of the semiconductor devices, relating to the third embodiment. In the NMOS transistor 50 shown in FIG. 4, one terminal of the resistor R3 is connected in common to back gates of respective NMOS transistors MN1, MN2 and MN3 of three NMOS transistor circuits 60, 70 and 80. The other terminal of the resistor R3 is connected to the ground. The structure of each of the NMOS transistors 60, 70 and 80 is the same as the conventional structure.

[0040] As explained above, according to the semiconductor device relating to the third embodiment, a resistor is used in common for the back gates of MOS transistors of a plurality of MOS transistor circuits. Therefore, it is possible to obtain the effects similar to those of the first embodiment. Further, as it is not necessary to provide a resistor to be connected to the back gate of each MOS transistor circuit, this structure is effective when there is no space for building a polysilicon resistor or a diffusion resistor corresponding to the plurality of transistors on the chip.

[0041] In the second and third embodiments, only the NMOS transistor has been explained. However, it is also possible to similarly apply the invention to the PMOS transistor circuit. Further, in the third embodiment, a polysilicon resistor, a diffusion resistor, or a wiring resistor can be used for the resistor that is connected to the back gate. In place of this resistor, it is also possible to use a high-impedance line circuit as explained in the second embodiment.

[0042] In the first to third embodiments, a MOS transistor circuit that is connected to the source (drain) of the MOS transistor, or that is connected directly to the ground, or that is connected to the power source terminal is explained. However, structure is not particularly limited to this. It is sufficient when the resistor or the high-impedance circuit is connected to the back gate of the MOS transistor as described above.

[0043] As explained above, according to the present invention, in a MOS transistor circuit having a surge protection circuit connected in parallel with a MOS transistor, impedance adding unit is provided at the back gate of the MOS transistor. Therefore, it is possible to apply an input impedance higher than an input impedance of the surge protection circuit to between the back gate of this MOS transistor and the ground (or the power source terminal). As a result, there is an effect that it is possible to prevent a surge current from flowing via the back gate of the MOS transistor, and it becomes possible to prevent the MOS transistor from being destroyed and short-lived.

[0044] Furthermore, in a plurality of MOS transistor circuits having a surge protection circuit connected in parallel with each MOS transistor, impedance adding unit connected in common to the back gate of each MOS transistor is provided. Therefore, one impedance adding unit can apply an input impedance higher than an input impedance of the surge protection circuit to between the back gate of each MOS transistor and the ground (or the power source terminal). As a result, there is an effect that it is possible to prevent a surge current from flowing via the back gate of the MOS transistor, and it becomes possible to prevent the MOS transistor from being destroyed and short-lived. Further, it is possible to minimize the increase in the area due to the addition of the impedance adding unit.

[0045] Furthermore, in a MOS transistor circuit having a surge protection circuit connected in parallel with a MOS transistor, there is an effect that it is possible to utilize a resistor formed by a semiconductor layer like a polysilicon resistor or a diffusion resistor as the impedance adding unit that is disposed between the back gate of the MOS transistor and the ground (or the power source terminal).

[0046] Furthermore, in a MOS transistor circuit having a surge protection circuit connected in parallel with a MOS transistor, it is possible to utilize a wiring resistor formed by wiring as the impedance adding unit that is disposed between the back gate of the MOS transistor and the ground (or the power source terminal). Therefore, there is an effect that it is possible to apply a high impedance to the back gate when there is no space for building a resistor formed by a semiconductor on the chip.

[0047] Furthermore, in a MOS transistor circuit having a surge protection circuit connected in parallel with a MOS transistor, a high impedance point of other semiconductor circuit is connected to a back gate of the MOS transistor. Therefore, it is possible to apply an input impedance higher than an input impedance of the surge protection circuit to between the back gate of this MOS transistor and the ground (or the power source terminal). As a result, there is an effect that it is possible to prevent a surge current from flowing via the back gate of the MOS transistor, and it becomes possible to prevent the MOS transistor from being destroyed and short-lived. Further, it is not necessary to provide a special circuit for applying this high impedance.

[0048] Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth. 

What is claimed is:
 1. A semiconductor device comprising: a MOS transistor having a source or a drain connected with a wire to an electrode pad; a surge protection circuit connected in parallel with the wire that connects said MOS transistor and said electrode pad; and an impedance adding unit that has input impedance higher than input impedance of said surge protection circuit and that is connected to a back gate of said MOS transistor.
 2. The semiconductor device according to claim 1, wherein said impedance adding unit is a resistor formed by a semiconductor layer.
 3. The semiconductor device according to claim 1, wherein said impedance adding unit is a wiring resistor.
 4. A semiconductor device comprising: a plurality of MOS transistors and same number of electrode pads as said MOS transistors, wherein each of said MOS transistor having a source or a drain connected with a wire to corresponding said electrode pad; same number of surge protection circuits as said MOS transistors, wherein each of said surge protection circuit being connected in parallel with the wire that connects corresponding said MOS transistor and said electrode pad; and an impedance adding unit that has input impedance higher than input impedance of each of said surge protection circuit and that is connected in common to back gates of said MOS transistors.
 5. The semiconductor device according to claim 4, wherein said impedance adding unit is a resistor formed by a semiconductor layer.
 6. The semiconductor device according to claim 4, wherein said impedance adding unit is a wiring resistor.
 7. A semiconductor device comprising: a MOS transistor having a source or a drain connected with a wire to an electrode pad; and a surge protection circuit connected in parallel with the wire that connects said MOS transistor and said electrode pad, wherein back gate of said MOS transistor is connected to a point of a semiconductor circuit at which point the impedance is higher than input impedance of said surge protection circuit. 